1. Field of the Invention
The present invention relates to a method and a means for allocating time slots in a time division duplex communication system. Particularly, the method and the means for allocating time slots according to the present invention can be implemented in communication units of a time division duplex communication system, e.g. a base station and/or a mobile station of a telecommunication system.
2. Description of Related Art
In a time division duplex communication system, examples of which are the GSM-standard for outdoor mobile communication or the DECT-standard for indoor mobile telephone communication, the receiving and the transmitting channels are separated on a time basis. E.g. in the DECT-standard, the information is transmitted in time frames of a length of 10 ms having 24 time slots. The first 12 time slots are used for the downlink connection, that is the transmission of data from a base station to a mobile station. The last 12 time slots are used for the uplink connection, that is for the transmission of data from a mobile station to the base station. In the GSM-standard, a combination of a frequency division duplex and a time division duplex is used. The information is transmitted in time frames having 8 time slots, whereby the uplink connection is provided in a lower frequency band and the downlink connection is provided in a higher frequency band. Each of the 8 time slots in each frame is assigned to a different mobile station. Thereby, the time slots assigned to a certain mobile station within a downlink time frame are separated from the time slots assigned to the same mobile station in the uplink time frame by two time slots. In other words, if e.g. the first time slot in a downlink time frame is assigned to a certain mobile station, the fourth time slot in an uplink time frame is assigned to the same mobile station. The time basis separation of the uplink and downlink time slots enables the construction of the mobile stations to be made more simple, since the reception and the transmission of data does not take place simultaneously.
Since in telecommunication systems as e.g. the GSM-system the data transfer rate is restricted, attempts have been made to increase the data transfer rate by allocating more than one time slot per frame to a certain mobile station. In JP 05153033-A such a TD digital mobile telecommunication system is disclosed, in which the same frequency for transmitting and receiving information is used. 1 to N time slots within one uplink time fame are allocated to one mobile station and 1 to N time slots within one downlink time frame are allocated to one mobile station depending on the information volume to be transferred between the mobile station and a base station. Each frame is allocated either to the uplink transfer of data or the downlink transfer of data. The uplink time frames cannot be used for a downlink transfer of data, so that a strong asymmetric transmission of information with a large difference between the amount of uplink data and the amount of downlink data is not possible.
In JP 07107546-A, a TDMA radio communication system is disclosed, in which the ratio between the number of uplink and downlink time slots within one time frame or one super frame consisting of several time frames is changed according to the total amount of traffic between a base station and mobile stations. In case of fast changing data transfer amounts, the switching point within each frame between the uplink time slots and the downlink time slots often changes position. Every change of such a switching point requires a reallocation of several time slots for the different connected mobile stations. This known system therefore requires a complicated circuitry.